In recent years, reductions in size and weight of mobile information terminals such as a cellular phone, a notebook-size personal computer, PDA, and the like have been rapidly advanced, and batteries used as driving power supplies have been required to have higher capacity. Lithium ion batteries which are charged and discharged by movement of lithium ions between positive and negative electrodes in association with charge and discharge have a high energy density and high capacity, and are thus widely used as driving power supplies for the above-described mobile information terminals.
The mobile information terminals are liable to be further increased in power consumption with enhancement of functions such as a video replay function and a game function, and are strongly demanded to have higher capacity. A method for increasing the capacity of the nonaqueous electrolyte batteries is, for example, a method of increasing the capacity of an active material, a method of increasing the amount of an active material filling per unit volume, or a method of increasing the charge voltage of a battery. However, an increase in charge voltage of a battery increases reactivity between a positive electrode active material and a nonaqueous electrolyte and degrades materials involved in charge and discharge of a battery, thereby not a little adversely affecting battery performance.
In order to solve the above problems, proposals described below have been made.
(1) A positive electrode active material is coated with a fluoride such as aluminum fluoride, zinc fluoride, lithium fluoride, or the like in an amount of 0.1 to 10% by weight in terms of metal atom relative to the weight of the positive electrode active material. It is described that cycling characteristics are improved as a result (refer to Patent Literature 1 below).
(2) A method for producing a positive electrode including mixing a fluoride at a ratio of 0.3 to 10% by weight relative to the weight of a positive electrode active material is described, in which a composite oxide as a raw material containing lithium, a transition metal, and oxygen is mixed with a fluoride of a rare earth element having an average particle diameter of 20 μm or less, and the resultant mixture is further ground and mixed (refer to Patent Literature 2 below).